Wood products, such as logs, boards, other lumber products, or the like, can be graded or classified into qualitative groups by the amount of warp potential, or dimensional stability, in the product. Crook, bow, twist, and cup are examples of warp and are illustrated in FIG. 1. The groups are used to qualitatively represent the warp state at a specified ambient condition or the degree of warp instability of a wood product. The qualitative groups are typically ordinal in nature, though nominal categories may also be used.
The degree of warp depends on several known factors, such as density, modulus of elasticity (hereinafter referred to as “MOE”), moisture content variation, pith location, compression wood, grain angle and others. Many of these factors can be quantitatively or qualitatively evaluated with different types of sensors. For example, MOE can be estimated from the propagation of sound through wood, and specific gravity can be estimated from the capacitance of wood. A different type of sensor group or system may be utilized for detecting each of these properties.
During the three year period from 1995 to 1998, solid sawn softwood lumber usage in wall framing, floor framing and roof framing dropped by 9.9%, 17.2% and 11% respectively in the United States (Eastin et al., 2001)1. In this survey of nearly 300 builders, lumber straightness was rated the most important factor affecting buying decisions; yet of all the quality attributes surveyed, dissatisfaction with straightness was highest. It is generally recognized that softwood lumber will continue to lose market share unless the industry improves the in-service warp stability of its product. 1Eastin, I. L., Shook, S. R., Fleishman, S. J., Material substitution in the U.S. residential construction industry, 1994 versus 1988, Forest Products Journal, Vol. 51, No. 9, 31-37.
In the United States, most softwood dimension lumber is visually graded for a variety of attributes that affect its appearance and structural properties. These attributes include knots, wane, dimension (thickness, width, and length), decay, splits and checks, slope-of-grain, and straightness (warp). Strict quality control practices overseen by third party grading agencies are in place to ensure that all lumber is “on-grade” at the point the grade is assigned. Unfortunately, the straightness and dimension of a piece are not static and can change after the piece is graded. Additional warp and size change can develop after the piece is in the distribution channel or after it is put into service. Typical moisture content of fresh kiln dried lumber averages 15% but ranges from 6% to 19%. This lumber will eventually equilibrate to a moisture ranging from 3% to 19% depending on time of year, geography and whether the application is interior or exterior (Wood Handbook)2. This moisture change results in changes in both dimension and warp properties. Any piece of lumber is prone to develop additional “in-service” warp if a) its shrinkage properties are not uniform and it changes moisture or b) its moisture content is not uniform at the point the original grade was assigned. Neither of these conditions is detectable with traditional visual grading methods. Customers of wood products seek stability in both dimension and warp properties. 2 Wood Handbook, General Technical Report 113 (1999) Department of Agriculture, Forest Service, Forest Products Laboratory.
The wood handbook2 provides guidelines for assessing the width and thickness stability of solid sawn lumber. Average thickness and width shrinkage is governed by grain orientation as well as radial and tangential shrinkage properties. These average radial and tangential shrinkage values vary by species and are reduced if heartwood is present. Although these methods can be used to estimate the average thickness and width shrinkage behaviour of a species, methods for precise quantification do not exist. There are even fewer design tools for estimating length shrinkage. 2 Wood Handbook, General Technical Report 113 (1999) Department of Agriculture, Forest Service, Forest Products Laboratory.
Today the patterns of equilibrium moisture and shrinkage coefficients within a full size lumber product can be accurately measured only in a laboratory environment. The laboratory technique involves cutting the piece of lumber into small “coupons” and measuring the moisture content and shrinkage coefficients using ASTM standards D-4492 and D-143, respectively. Although much is known about equilibrium moisture and shrinkage behaviour of wood, there are as yet no comprehensive theoretical models and no methods of monitoring these properties in a real time production environment.
However, it has been empirically noted that the amount of warp in a lumber product at one moisture content or environmental condition is typically highly correlated with the warp of the same piece of lumber at a different moisture content. In some applications this correlation may be sufficient to identify pieces of lumber that are projected to meet certain end-use requirements for dimensional stability